1. Field of the invention
The invention is in the field of transmission of digital data by optical means. It is more particularly concerned with transmission at high bit rates over long-haul fiber optic links.
2. Description of the prior art
In this case transmission is effected by means of an optical transmitter connected to an optical receiver by the fiber. The transmitter modulates the power of an optical carrier wave in accordance with the information to be transmitted. The modulation usually consists in varying the power of the carrier wave between two levels: a low level corresponding to extinction of the wave and a high level corresponding to the maximum optical power of a laser oscillator. By convention, the low and high levels respectively represent the binary values "0" and "1". The level variations of the wave are triggered at times imposed by a clock signal which in this way defines successive time cells allocated to the data to be transmitted.
The maximal transmission distance is generally limited by the ability of the receivers to detect these two power levels without errors after propagation of the modulated wave in the optical link. To increase this distance, an attempt is generally made to increase the ratio between the optical power of the high level and that of the low level, this ratio defining the "extinction rate" which is one of the characteristics of the modulation.
Moreover, for a given distance and a given extinction rate, the information bit rate is limited by the chromatic dispersion caused by the fibers. This dispersion is caused by the dependence of the effective index of the fiber on the wavelength of the wave transported and one consequence of this is that the width of the transmitted pulses increases as they propagate along the fiber.
One proposal to limit the consequences of this phenomenon is to reduce the bandwidth of the signal to be transmitted by means of appropriate coding. One particular proposal is to use the "duobinary" code that is well known in the field of electrical transmission. This code has the property of halving the signal bandwidth. The code uses a signal with three levels respectively symbolized 0, + and -. The binary value 0 is coded by the 0 level and the value 1 is coded either by the + level or by the - level, with an encoding rule whereby the levels coding two successive blocks of "1" surrounding a respectively even or odd number of consecutive "0" are respectively identical or different.
The use of the duobinary code for optical transmission is mentioned in the article: "10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fibre", X. Gu et al, ELECTRONICS LETTERS, 9 December 1993, Vol. 29, N.sup.o 25. In this article, the three levels 0, +, - respectively correspond to three optical power levels.
U.S. patent application SN 08/557,184 also describes duobinary coding applied to optics. In this document, the binary value "0" always corresponds to a low level of the optical power and the symbols + and - correspond to the same high optical power level and are distinguished by a 180.degree. phase-shift of the optical carrier.
The use of this latter phase inverting duobinary code is also mentioned in the article: "Optical duobinary transmission system with no receiver sensitivity degradation", K. Yonenaga et al, ELECTRONICS LETTERS, 16 February 1995, Vol. 31, N.sup.o 4.
Although these reports of experiments with this code indicate an improvement compared to the conventional NRZ ("no return to zero") code, this improvement is not always obtained. On approaching ideal conditions for use of this code, in particular the highest possible extinction rate, the maximal improvement should be obtained. Paradoxically, simulations and tests have given results which are the opposite of those expected.
If the physical effects of the duobinary code in the context of fiber optics are analyzed in detail, it can be shown that a reduction in the bandwidth of the signal is indeed obtained. On the other hand, this code has no influence on the spectrum of each pulse considered in isolation, whereas it is precisely this spectrum which is decisive in the matter of the effects of chromatic dispersion.
The positive results reported by various articles are difficult to explain. Although some experimental parameters can be verified (length and quality of the fiber, bit rate), other parameters cannot be accurately controlled: characteristics of the optical components and the real operation of the control electronic circuits.
Simulations and tests in which the experimental parameters were varied, indicated that an improvement is obtained provided that a phase-shift of the carrier wave is produced within each "0" which precedes or succeeds each block of "1" or each isolated "1". Moreover, it is not necessary for the low power level that codes the "0" to be the lowest possible, in other words for the extinction rate to be the highest possible. In practise an optimal value of the extinction rate is a complex function of the other experimental parameters.
In view of the above, an aim of the invention is to propose an optical transmission method exploiting the observations reported above. This method could be called the "phase-shaped binary transmission" (PSBT) method.